This invention relates generally to a method of bonding semiconductor devices to heatsinks and specifically relates to a method which provides a more uniform, parallel pressure between a device and a heatsink in a thermocompression bonding process.
It is known to use thermocompression bonding to bond semiconductor devices to headers and heatsinks in the semiconductor art. Often, to facilitate thermocompression bonding, thick metallic films, called bonding pads, are deposited on the surfaces to be joined. The bonding pads are typically of a malleable metal or alloy. When the two bodies are pressed together, the bonding pads compress and are fused by the appropriate combination of temperature, pressure and time. The malleable nature of the pad can also serve to absorb some of the stress of the pressure applied during the process. Thermocompression bonding provides bonding of two objects at a temperature lower than the melting point of the pad material and without the use of a flux.
A problem with bonding small semiconductor devices to heatsinks is the susceptibility of the device to damage or breakage from the pressure applied. For example, electroluminescent devices of gallium arsenide, indium phosphide and the like, because of their inherent strain, can only withstand limited amounts of stress before crystalline damage occurs. Often the pressures required to bond these devices to heatsinks using high-quality gold or gold alloy bonding pads approach the stress limits for the crystalline materials. This damage adversely affects the life of the device.
Even when the bonding is carried out at pressures below the stress limits for a given crystal, damage can result. Misalignment of the device and the heatsink can concentrate all of the pressure in one small area and this point pressure can far exceed the stress limits for the crystalline device. Excessive pressure is also exerted on nodules or high points on the bonding pads.
It therefore becomes important to apply a uniform, parallel pressure to the bonding pads during the bonding process. Providing bonding pads with smooth, defect-free surfaces is the first step towards accomplishing this purpose.
A copending application entitled "Method of Burnishing Malleable Films on Semiconductor Substrates" by F. Z. Hawrylo was filed concurrently herewith and is incorporated herein by reference. The copending application describes a burnishing process suitable for providing substantially defect-free surfaces on the bonding pads of semiconductor devices without damaging the fragile devices.
The next step is aligning the two bonding pad surfaces (device and heatsink) so that they are substantially parallel. Even defect-free surfaces, if not parallel to each other, will cause point pressure, i.e. a highly localized excessive pressure, during thermocompression bonding.
When considering the size of the bonding area, about 7.times.12 mils, it becomes apparent that even the slightest degree of non-parallelism or defects can be a problem. Fine tuning the parallel alignment of each device to its heatsink in a production environment is not practical. Besides being time consuming, this would make the bonding surfaces parallel but would not eliminate surface defects. Likewise, a prior polishing process for heatsink and devices would serve to eliminate surface defects but would not insure parallelism of the bonding pads. Finally, mechanical scrubbing, a process in which the two bonding pads are rubbed together prior to bonding designed to make the bonding pad surfaces more compatible, typically causes exactly the type of substrate damage which is sought to be avoided during bonding. Reliable thermocompression bonding of 4 mil-thick GaAs and InP devices requires that the surfaces of the heatsink pad and device pad be as near perfect a match as possible.
It would be desirable, therefore, to have a method of custom-matching the two bonding surfaces to maximize the uniformity of the pressure applied during the subsequent thermocompression bonding steps.